The Red Swordtail Fish, scientifically known as Xiphophorus hellerii, belongs to the Poeciliidae family. Originating from Central America, particularly Mexico, these freshwater fish are renowned for their vibrant red coloration and distinctive sword-like extension of the lower part of their tails in males. Swordtails are livebearers, meaning they give birth to live fry rather than laying eggs, contributing to their popularity in the aquarium hobby.

Creating an optimal environment for Red Swordtail Fish involves:

- **Tank Size:** Provide a tank of at least 20 gallons for a small group of Swordtails.
- **Water Parameters:** Maintain slightly alkaline water with a pH of 7.0-8.0.
- **Temperature:** Keep the water temperature between 72-82°F (22-28°C).
- **Filtration:** Use a reliable filter to ensure good water quality.
- **Decor:** Include plants and hiding places, as well as open swimming areas.

Companion species for Red Swordtail Fish include:

Fish:
1. **Platy (Xiphophorus maculatus):** Peaceful livebearer, similar care requirements.
2. **Guppy (Poecilia reticulata):** Colorful, compatible in a community setting.
3. **Mollies (Poecilia spp.):** Varied species, adaptable to similar water conditions.
4. **Angelfish (Pterophyllum scalare):** Taller fish, complements Swordtails.
5. **Corydoras Catfish (Corydoras spp.):** Bottom-dwelling, complements the mid and upper levels.

Crustaceans:
1. **Red Cherry Shrimp (Neocaridina davidi):** Hardy and colorful addition.
2. **Amano Shrimp (Caridina multidentata):** Effective algae eater, peaceful cohabitant.
3. **Ghost Shrimp (Palaemonetes spp.):** Inexpensive and compatible scavengers.
4. **Bamboo Shrimp (Atyopsis moluccensis):** Filter feeder, non-aggressive.
5. **Mexican Dwarf Crayfish (Cambarellus patzcuarensis):** Small and suitable for community tanks.

Bugs:
1. **Daphnia:** Small crustaceans, excellent live food for Swordtails.
2. **Mosquito Larvae:** Natural and nutritious live food option.
3. **Wingless Fruit Flies (Drosophila melanogaster):** Suitable for smaller fish as a live food source.
4. **Brine Shrimp:** Enriched with nutrients, suitable for varied diet.
5. **Bloodworms:** High in protein, a favorite of many small fish.

Plants:
1. **Amazon Sword (Echinodorus spp.):** Adds greenery without being disruptive.
2. **Java Moss (Taxiphyllum barbieri):** Provides hiding spots and complements the environment.
3. **Anubias:** Hardy and can tolerate a variety of water conditions.
4. **Hornwort (Ceratophyllum demersum):** Floating plant, provides shade and cover.
5. **Cryptocoryne:** Hardy and provides hiding spots.

Amphibians:
1. **African Dwarf Frog (Hymenochirus spp.):** Peaceful cohabitant in a community tank.
2. **Fire-Bellied Toad (Bombina spp.):** Compatible in terms of size and habitat.
3. **African Clawed Frog (Xenopus laevis):** Aquatic, doesn't disturb Swordtails.
4. **Dart Frogs (Dendrobatidae):** Terrestrial, won't interfere with the aquarium's balance.
5. **Mudskippers (Periophthalmus spp.):** Brackish water species, unique addition.

These carefully selected companions contribute to a balanced ecosystem, promoting the well-being of Red Swordtail Fish while enhancing the overall aesthetics of the aquarium.